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Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (July 25, 2018) is 1389 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Unused Reference: 'RFC6125' is defined on line 326, but no explicit reference was found in the text == Outdated reference: draft-ietf-acme-acme has been published as RFC 8555 ** Obsolete normative reference: RFC 3501 (Obsoleted by RFC 9051) ** Obsolete normative reference: RFC 5246 (Obsoleted by RFC 8446) Summary: 3 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Melnikov 3 Internet-Draft Isode Ltd 4 Intended status: Informational July 25, 2018 5 Expires: January 26, 2019 7 Extensions to Automatic Certificate Management Environment for email TLS 8 draft-ietf-acme-email-tls-05 10 Abstract 12 This document specifies identifiers and challenges required to enable 13 the Automated Certificate Management Environment (ACME) to issue 14 certificates for use by TLS email services. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at https://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on January 26, 2019. 33 Copyright Notice 35 Copyright (c) 2018 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (https://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Conventions Used in This Document . . . . . . . . . . . . . . 2 52 3. Use of ACME for use by TLS-protected SMTP, IMAP and POP3 53 services . . . . . . . . . . . . . . . . . . . . . . . . . . 2 54 3.1. "service" field in JSON payload . . . . . . . . . . . . . 3 55 3.2. "port" field in JSON payload . . . . . . . . . . . . . . 4 56 3.3. DNS challenge for email services . . . . . . . . . . . . 4 57 3.4. CAPABILITY challenge for email services . . . . . . . . . 4 58 3.4.1. Registration of the ACME SMTP extension . . . . . . . 6 59 4. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 6 60 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 61 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 62 7. Normative References . . . . . . . . . . . . . . . . . . . . 7 63 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 65 1. Introduction 67 [I-D.ietf-acme-acme] is a mechanism for automating certificate 68 management on the Internet. It enables administrative entities to 69 prove effective control over resources like domain names, and 70 automates the process of generating and issuing certificates. 72 This document describes extensions to ACME for use by email services. 73 Section 3 defines extensions for how email services (such as SMTP, 74 IMAP and POP3) can get certificates for use with TLS. 76 2. Conventions Used in This Document 78 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 79 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 80 document are to be interpreted as described in [RFC2119]. 82 3. Use of ACME for use by TLS-protected SMTP, IMAP and POP3 services 84 SMTP [RFC5321] (including SMTP Submission [RFC6409]), IMAP [RFC3501] 85 and POP3 [RFC2449] servers use TLS [RFC5246] to provide server 86 identity authentication, data confidentiality and integrity services. 87 Such TLS protected email services either use STARTTLS command or run 88 on a separate TLS-protected port [RFC8314]. 90 [I-D.ietf-acme-acme] defines several challenge types that can be 91 extended for use by email services. This document also defines some 92 new challenge types specific to SMTP, IMAP and POP3. 94 In order to use these challenges JWS [RFC7515] object used by 95 [I-D.ietf-acme-acme] is extended. The following extra requirements 96 are in addition to requirements on JWS objects sent in ACME defined 97 in Section 6.2 of [I-D.ietf-acme-acme]: 99 1. "service" JWS header parameter MUST be included. See Section 3.1 100 for more details. 102 2. "port" JWS header parameter SHOULD be included. See Section 3.2 103 for more details. If this JWS header parameter is not included, 104 the default assigned IANA port for the corresponding "service" is 105 assumed. 107 For example, if the ACME client were to respond to the "dns-email-00" 108 challenge, it would send the following request: 110 POST /acme/authz/asdf/0 HTTP/1.1 111 Host: example.com 112 Content-Type: application/jose+json 114 { 115 "protected": base64url({ 116 "alg": "ES256", 117 "kid": "https://example.com/acme/acct/1", 118 "nonce": "Q_s3MWoqT05TrdkM2MTDcw", 119 "url": "https://example.com/acme/authz/asdf/0" 120 }), 121 "payload": base64url({ 122 "type": "dns-email-00", 123 "service": "smtp", 124 "port": 25, 125 "keyAuthorization": "IlirfxKKXA...vb29HhjjLPSggQiE" 126 }), 127 "signature": "7cbg5JO1Gf5YLjjF...SpkUfcdPai9uVYYU" 128 } 130 Figure 1 132 3.1. "service" field in JSON payload 134 The "service" field in JSON payload specifies the service for which 135 TLS server certificate should be issued. Valid values come from 136 "Service Names and Transport Protocol Port Numbers" IANA registry 137 . 140 ACME servers compliant with this specification MUST support [RFC7817] 141 (in particular see Section 4 of that document). 143 [[This parameter might have applicability beyond email services.]] 145 3.2. "port" field in JSON payload 147 The "port" field in JSON payload specifies the TCP port number where 148 the corresponding service is running. ACME server MAY check that the 149 TCP port corresponds to the requested "service", for example that the 150 port is the assigned default IANA port for the service. 152 [[This parameter might have applicability beyond email services.]] 154 3.3. DNS challenge for email services 156 "dns-email-00" is very similar to "dns-01" defined in Section 8.4 of 157 [I-D.ietf-acme-acme]. 159 The difference between processing of "dns-email-00" and "dns-01" are 160 listed below: 162 1. The TXT record used to validate this challenge is 163 _._._acme-challenge.. For example, for 164 domain "example.com" and IMAPS service running on port 993, the 165 TXT record name is _993._imaps._acme-challenge.example.com. For 166 domain "example.net" and IMAP service running on port 143, the 167 TXT record name is _143._imap._acme-challenge.example.next. 169 3.4. CAPABILITY challenge for email services 171 For "capability-smtp-00" challenge, ACME client (== SMTP server) 172 constructs a key authorization from the "token" value provided in the 173 challenge and the client's account key. The client then computes the 174 SHA-256 digest [FIPS180-4] of the key authorization. SMTP server 175 than returns the base64url encoding of this digest as a value of the 176 "ACME" EHLO capability. For example: 178 250-smtp.example.com 179 250-SIZE 180 250-8BITMIME 181 250-BINARYMIME 182 250-PIPELINING 183 250-HELP 184 250-DSN 185 250-CHUNKING 186 250-AUTH SCRAM-SHA-1 187 250-AUTH=SCRAM-SHA-1 188 250-STARTTLS 189 250-ACME gfj9Xq...Rg85nM 190 250-MT-PRIORITY 191 250 ENHANCEDSTATUSCODES 193 Note that in the above example only presence of the ACME is relevant 194 as far as this document is concerned. 196 Figure 2 198 The ACME SMTP extension is formerly defined in Section 3.4.1. 200 Similarly, "capability-imap-00" challenge, ACME client (== IMAP 201 server) constructs a key authorization from the "token" value 202 provided in the challenge and the client's account key. The client 203 then computes the SHA-256 digest [FIPS180-4] of the key 204 authorization. IMAP server than returns the base64url encoding of 205 this digest as a value of the "ACME" capability: 207 * OK [CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM] Example IMAP4rev1 server ready 209 or 211 * CAPABILITY IMAP4rev1 LOGINDISABLED LITERAL+ ENABLE STARTTLS ACME=gfj9Xq...Rg85nM 213 Note that in the above example only presence of the ACME capability 214 token is relevant as far as this document is concerned. 216 Figure 3 218 Similarly, "capability-pop-00" challenge, ACME client (== POP3 219 server) constructs a key authorization from the "token" value 220 provided in the challenge and the client's account key. The client 221 then computes the SHA-256 digest [FIPS180-4] of the key 222 authorization. POP3 server than returns the base64url encoding of 223 this digest as a value of the "ACME" capability in response to CAPA 224 command [RFC2449]: 226 C: CAPA 227 S: +OK Capability list follows 228 S: TOP 229 S: SASL CRAM-MD5 KERBEROS_V4 230 S: UIDL 231 S: ACME gfj9Xq...Rg85nM 232 S: IMPLEMENTATION Shlemazle-Plotz-v915 233 S: . 235 Note that in the above example only presence of the ACME capability 236 token is relevant as far as this document is concerned. 238 Figure 3 240 3.4.1. Registration of the ACME SMTP extension 242 The ACME SMTP service extension is defined as follows: 244 1. The textual name of this extension is "ACME for SMTP". 246 2. The EHLO keyword value associated with this extension is "ACME". 248 3. The EHLO keyword has a single required parameter which is a 249 base64url encoded SHA-256 hash, which is 44 octets in length. 251 4. This extension doesn't define any new SMTP verbs. 253 5. This extension doesn't add any new parameters to MAIL FROM or 254 RCPT TO commands. 256 6. The ACME extension is valid for the submission service [RFC6409] 257 (default port number 587) or its version running directly over 258 TLS [RFC8314] ("submissions" service, default port number 465) . 260 4. Open Issues 262 [[This section should be empty before publication]] 264 1. Should the same certificate be allowed to be used on both IMAP 265 (143) and IMAPS (993) ports? (These ports have different service 266 names associated with them. Is 1 service/port per ACME 267 certificate a restriction imposed by this document?) Maybe if 268 the ACME server sees a request for port 143 (or 993), it can 269 include SRV-ID for the other port, if it can verify that both are 270 running? (How can this be done reliably?) Many email servers 271 don't allow different certificates to be configured for different 272 ports they are listening on. The cleanest way is to change 273 "service" to "services", change "port" to "ports" and make both 274 of them arrays. 276 2. Add support for LMTP (RFC 2033)? 278 5. IANA Considerations 280 IANA is requested to register the following ACME challenge types that 281 are used with Identifier Type "dns": "dns-email", "capability-smtp", 282 "capability-imap" and "capability-pop". The reference for all of 283 them is this document. 285 6. Security Considerations 287 Security Considerations from [I-D.ietf-acme-acme] relevant to the DNS 288 challenge type are also relevant to "dns-email". 290 7. Normative References 292 [FIPS180-4] 293 National Institute of Standards and Technology, "Secure 294 Hash Standard (SHS)", FIPS PUB 180-4, August 2015, 295 . 298 [I-D.ietf-acme-acme] 299 Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. 300 Kasten, "Automatic Certificate Management Environment 301 (ACME)", draft-ietf-acme-acme-12 (work in progress), April 302 2018. 304 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 305 Requirement Levels", BCP 14, RFC 2119, 306 DOI 10.17487/RFC2119, March 1997, 307 . 309 [RFC2449] Gellens, R., Newman, C., and L. Lundblade, "POP3 Extension 310 Mechanism", RFC 2449, DOI 10.17487/RFC2449, November 1998, 311 . 313 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 314 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 315 . 317 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 318 (TLS) Protocol Version 1.2", RFC 5246, 319 DOI 10.17487/RFC5246, August 2008, 320 . 322 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 323 DOI 10.17487/RFC5321, October 2008, 324 . 326 [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and 327 Verification of Domain-Based Application Service Identity 328 within Internet Public Key Infrastructure Using X.509 329 (PKIX) Certificates in the Context of Transport Layer 330 Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 331 2011, . 333 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 334 STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011, 335 . 337 [RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web 338 Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May 339 2015, . 341 [RFC7817] Melnikov, A., "Updated Transport Layer Security (TLS) 342 Server Identity Check Procedure for Email-Related 343 Protocols", RFC 7817, DOI 10.17487/RFC7817, March 2016, 344 . 346 [RFC8314] Moore, K. and C. Newman, "Cleartext Considered Obsolete: 347 Use of Transport Layer Security (TLS) for Email Submission 348 and Access", RFC 8314, DOI 10.17487/RFC8314, January 2018, 349 . 351 Author's Address 353 Alexey Melnikov 354 Isode Ltd 355 14 Castle Mews 356 Hampton, Middlesex TW12 2NP 357 UK 359 EMail: Alexey.Melnikov@isode.com